Fixed, the problem indeed was that some differences went into completely dark/black areas so was impossible to spot them!
I wonder how other people managed to solve it. Unless is a bug I added in an update. Anyway, I've uploaded the new version on my site (same download link, I don't bother to use version numbers for this game).

Since this a similar subject and I don't want to start a new thread, I found a similar problem in another "spot the difference mini-game".

Spoiler:

This time it's the Ragni murder scene. Depending on the shape, numbers 2,3, and 7 are almost impossible to find since they hide under the the black masses. 1 is borderline (it depends on the shape of the random object)

Last edited by Jaeger on Wed Feb 05, 2014 1:10 am, edited 1 time in total.